Much of the recent development of early transition and f-element hydrocarbyl and hydride chemistry can be attributed to the beneficial characteristics of bis(pentamethylcyclopentadienyl), [.eta..sup.5 (CH.sub.3).sub.5 C.sub.5 ].sub.2 (hereinafter Cp.sub.2 ') supporting ligation. However, in studies of isoelectronic, isoleptic 4f/5f systems, it appears that synthesis routes to lanthanide Cp.sub.2 'MR and (Cp.sub.2 'MH).sub.2 complexes were circuitous for late lanthanides and unknown for early (La-Nd) lanthanides. The latter appear of greatest interest since U(III).sup.6 and Nd(III).sup.5 are isoelectronic, because the early organolanthanides have the greatest importance in catalysis, and because, a priori, the early lanthanides might, for a given ligand array, offer the greatest degree of coordinative unsaturation and possibly reactivity. In addition, the early lanthanides are easily available and inexpensive. In planning syntheses, the use of highly lipophilic, sterically bulky R functionalities was anticipated so that undesirable coordination to the lanthanide ion of ether or halide ligands (normally present during the preparation of Cp.sub.2 'MR complexes) could be avoided. In most d-element hydrocarbyl syntheses, the choice of R is also frequently dictated by the desire to avoid destabilizing .beta.-hydrogen atoms. However, it now appears that .beta.-alkyl elimination (e.g. eq.(1)) may be an equally important ##STR1## decomposition pathway for some organo-f-element complexes and must also be considered in ligand selection.
Bulky ligands such as CH[Si(CH.sub.3).sub.3 ].sub.2, hereinafter (CHTMS.sub.2), offer both the attraction of substantial lipophilic bulk, a lack of--hydrogen atoms, as well as a defense (heretofore largely unappreciated) against--alkyl elimination since such an elimination process would produce a relatively high energy Si.dbd.C bond (e.g., eq. (2)). In the present invention, this ligand is ##STR2## employed to straight-forwardly synthesize a broad class of stable Cp.sub.2 'MR complexes ranging from the lightest (M=La) to the heaviest (M=Lu) lanthanide. Other bulky ligands such as the mesitylene ligand afford similar properties. With both ligands, hydrogenolysis is shown to readily yield the corresponding series of hydride dimers, (Cp.sub.2 'MH).sub.2 for M=La to Lu. The rather extraordinary reactivity of these molecules is demonstrated herein by their very high activity in olefin oligomerization and polymerization. Also shown is the surprisingly high activity of the catalysts of the subject invention for olefin hydrogenation.
Mechanistically, homogeneous olefin hydrogenation catalysts with early transition metal, lanthanide, and actinide catalysts is not well understood, and offers a much different mechanistic situation than conventional Group VIII or late transition element catalysts. In particular, the metal center may be in a relatively high (.gtoreq.3) formal oxidation state, and/or it may not possess energetically accessible formal oxidation states for oxidative addition/reductive elimination processes, and/or it may be engaged in relatively polar metal-ligand bonding with a strong preference for "hard" ligands (not olefins), and may exhibit unusual M--H/M--C bond disruption enthalpy relationships vis-a-vis middle and late transition elements. Lanthanide ions represent the extreme cases of many of the above considerations, and as such can offer an opportunity to better understand hydrogenation catalysis in such environments.